Loop antenna circuit useful in subterranean tool

ABSTRACT

A loop antenna useful, for example, in investigation of earth formations. Embodiments of the invention provide a loop antenna circuit comprising a loop antenna disposed to generate, in response to an electromagnetic wave, a pick-up signal on an output node. Loop antenna circuit further includes a tuning network coupled to the loop antenna. The tuning network is disposed to provide simultaneous tuning at a plurality of interrogation frequencies. Further embodiments include a preamplifier circuit coupled to the loop antenna. The preamplifier circuit is disposed to receive the pick-up signal on an input node and provide a high impedance load to the loop antenna for a first of the plurality of interrogation frequencies to reduce the secondary radiation from the loop antenna to below a predetermined value at the said first interrogation frequency.

RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.10/427,899, filed May 1, 2003.

FIELD OF THE INVENTION

This invention relates to the field of well logging; in particular, itrelates to electromagnetic wave propagation systems to measure anattribute which relates to at least one of the borehole and surroundingformation; and more particularly, it relates to antenna circuits thatare deployable within a borehole to measure one or more attributes of anelectromagnetic wave as it passes.

BACKGROUND OF THE INVENTION

In the practice of logging-while-drilling (LWD),measurement-while-drilling (MWD) and wireline logging, it is well knownthat by studying the propagation characteristics of an electromagneticwave, useful clues regarding the characteristics of the earth formationscan be derived. To probe sections of the earth surrounding the borehole,a pair of transmitters can be positioned within a well borehole toradiate an electromagnetic field at a particular interrogationfrequency. This electromagnetic wave is influenced by theelectromagnetic energy shed back from the formation. A pair of spaceddifferential loop antenna receivers is conventionally positioned withinthe borehole to measure, for example, the attenuation and/or phase shiftof the electromagnetic wave as it passes between each receiver loopantenna. Various methods for analyzing the measurements to deriveestimates of certain characteristics of the earth formations surroundingthe well borehole are well known.

It is of increasing importance in oil and gas exploration to obtainaccurate and reliable measurements of an electromagnetic waveinvestigating a formation. However, the accuracy of the informationderived from the measurements can be degraded by the effects of magneticfield mutual cross-coupling between receiving loop antennae. Receivercross-coupling typically results from significant circulatingalternating current that is induced in a receiver loop antenna inresponse to an electromagnetic wave. This alternating current tends toproduce a secondary electromagnetic field that can have a corruptinginfluence on the primary electromagnetic wave generated by atransmitter. The secondary electromagnetic field will affect themeasurements obtained by any other receiving loop antenna in closeproximity to the first receiver, producing an error component due to thecross-coupling. The receiving antenna closest to an active transmittertends to receive a stronger signal and produce greater circulatingcurrents than subsequently spaced receiving antennae. Accordingly, themagnitude of the undesirable secondary electromagnetic field radiated byan antenna tends to be greater from the receiving antenna that is closerto an active transmitter, and the magnitude of the error component dueto cross-coupling tends to be higher in the next subsequently spacedreceiving antenna.

Considerable effort has been expended by the industry to compensatemeasurements for the cross-coupling error. For example, one known methodincludes a calibration procedure where, under laboratory conditions, thecross-coupling error for each frequency of interest is measured andstored. Thereafter, each subsequently measured value is adjustedaccordingly. Such methods tend to be cumbersome, may introduce newsources of error and may create maintenance restrictions. For example,extra components may be needed on the receiver circuits to simulate thecross-coupling effect in the lab. Parameters such as the distancebetween receivers, which will vary with temperature, are critical to theaccuracy of the estimate of the cross-coupling error. In addition, thecalibration may be invalidated by the replacement, in a receiver antennasystem, of a failed component that influences the cross-coupling.

Eliminating the source of the cross-coupling error in downhole tools hasproven to be problematic in the industry. For example, methods commonlyemployed to counter the excessive signal loss resulting from a lengthycoaxial cable, typically having a characteristic impedance less than 100ohms, involve matching the receiver loop antenna impedance to theimpedance of its load. However, the matching of receiver loop antennaimpedance to that of its load typically results in significantcirculating currents being induced in receiver loop that create receivercross-coupling. Other methods employed by prior art systems includespacing receiver antennae as far apart as possible to reduce the effectsof cross-coupling, such as locating each of a pair of receiver antennaeon opposing sides of a pair of transmitters.

A strong pick up signal is an important consideration in obtainingaccurate measurements. Prior art downhole tools that match the receiverantenna impedance to a load comprising a lengthy coaxial cable tend toemploy single turn antennae, even though a multiple turn loop antennatypically provides the advantages of a strong pickup as compared tohaving a single turn antenna. A multiple turn loop antenna, in the rangeof 6 to 12 inches diameter, commonly exhibits several hundred ohms ofimpedance at 2Mhz. Thus, prior art methods for matching the receiverantenna to the load impedance combined with the use of a step-uptransformer, tend to limit a receiver loop antenna to no more than asingle turn.

A further limitation of prior art receiver loop antenna systems is theirinability to be simultaneously series tuned at multiple interrogationfrequencies. It well known that it is advantageous to utilize multipleinterrogation frequencies to probe earth formations with electromagneticwaves. Certain attributes of the earth formation are discoverable onlywhen the interrogation frequency is of a specific range. Lowerfrequencies are able to investigate deeper regions of the earth for agiven transmitter and receiver spacing. Also, lower frequencies oftenmitigate borehole effects. Higher frequencies yield higher phase shiftand attenuation values for a given resistivity, which is advantageousfor increased accuracy in highly resistive formations of commercialinterest. In LWD and MWD systems where measurements are commonlyobtained while the measuring tool is rotating and moving axially throughthe borehole, greater and more accurate information about thesurrounding earth formation can be derived by obtaining simultaneousmeasurements of a plurality of interrogation frequencies.

There is therefore a need in the art for receiver systems, deployablewithin a borehole, that can utilize a loop antenna having multipleturns, as opposed to a single turn, to enable the antenna to pick-up astrong signal from which a more accurate measurement of particularattributes of an electromagnetic wave can be derived. Also, there is aneed for a loop antenna that can be simultaneously series tuned atplurality of interrogation frequencies to enable it to simultaneouslyand accurately pick-up the plurality of interrogation frequencycomponents from an electromagnetic wave. In addition, there is a needfor a method for decreasing the design, manufacture, and maintenancecost of systems, that deploy a pair of loop antenna receivers downhole,while still diminishing the undesirable effects of mutualcross-coupling. Furthermore, there is an ever present need for downholeantenna systems that are stable over a wide range of temperatures, andthat provide increasingly accurate and greater amounts of informationabout the earth formations surrounding a borehole.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a loop antenna circuitis provided for use in a borehole. The loop antenna circuit includes aloop antenna disposed to generate, in response to an electromagneticwave, a pick-up signal on an output node thereof, the pick-up signalrepresentative of, for each of a plurality of interrogation frequencies,relative magnitude and relative phase of the electromagnetic wave. Theloop antenna circuit further includes a tuning network electricallyconnected to the loop antenna, the tuning network configured to tune theloop antenna such that a combined source impedance of the loop antennaand the tuning network varies with frequency, the combined sourceimpedance having minima within a preselected band of frequencies abouteach of the interrogation frequencies.

In accordance with another aspect of the invention, a receiver isprovided for a downhole tool. The receiver includes a loop antenna, atuning network electrically connected in series with the loop antenna,and a preamplifier circuit having an input electrically connected inseries with the loop antenna and the tuning network. The loop antenna isdisposed to receive an electromagnetic wave in a borehole and includesat least a single turn of antenna wire deployed about a tool housing.The tuning network is configured such that a series impedance of theloop antenna and the tuning network is less than the impedance of theloop antenna in a preselected frequency band about at least oneinterrogation frequency. The preamplifier circuit has an input impedanceselected such that a series impedance of the loop antenna, the tuningnetwork, and the preamplifier circuit is sufficiently high to reducesecondary radiation from the loop antenna to below a predetermined valueat the at least one interrogation frequency.

In accordance with still another aspect of this invention, a loggingwhile drilling tool is provided for use in a subterranean borehole. Thelogging while drilling tool includes a housing adapted to be coupled toa drill string and deployed in a subterranean borehole. A transmitter isdeployed on the housing, the transmitter disposed to transmit anelectromagnetic wave into the borehole, the electromagnetic waveincluding a plurality of interrogation frequency components. First andsecond, longitudinally spaced loop antennas are disposed to receive theelectromagnetic wave, each of the loop antennas having at least a singleturn of antenna wire deployed about the tool body. The logging whiledrilling tool further includes a tuning network electrically connectedin series to the first loop antenna. The tuning network is configuredsuch that a series impedance of the loop antenna and the tuning networkis less than the impedance of the loop antenna in a preselectedfrequency band about each of said plurality of interrogationfrequencies.

It is therefore a technical advantage of the invention is to provideantenna receiver systems to measure the amplitude and phase of aplurality of interrogation frequencies included in an electromagneticwave with improved accuracy as compared with prior art systems.Additionally, the present invention provides cost effective and spaceefficient loop antenna receivers that work reliably in the adverseconditions commonly found while drilling in a subterranean borehole.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand the specific embodiments disclosed may be readily utilized as abasis for modifying or designing other structures for carrying out thesame purposes of the present invention. It should also be realized bythose skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the invention as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 depicts, in block diagram form, an exemplary measuring tool onwhich the present invention may be deployed;

FIG. 2A is a schematic block diagram of an exemplary embodiment of thepresent invention;

FIG. 2B is a schematic block diagram of an exemplary embodiment of thepresent invention;

FIGS. 2C and 2D are plots of impedance versus frequency responses foralternative embodiments of the present invention;

FIG. 2E is a schematic block diagram of an exemplary embodiment of thepresent invention;

FIG. 2F is a plot of impedance versus frequency responses for anexemplary embodiment of the present invention;

FIG. 2G is a schematic block diagram of an exemplary embodiment of thepresent invention;

FIG. 2H and 2I are plots of impedance and gain responses, respectively,of an exemplary embodiment of the present invention;

FIG. 3 is a schematic of an embodiment of a tuning network in accordancewith the present invention;

FIG. 4 is a schematic of an alternative embodiment of a tuning networkin accordance with the present invention;

FIG. 5 is a schematic of a further alternative embodiment of a tuningnetwork in accordance with the present invention;

FIG. 6A and 6B are schematics of an embodiment of a preamplifier circuitin accordance with the present invention; and

FIG. 7 is a schematic of an alternative embodiment of a preamplifiercircuit in accordance with the present invention.

DETAILED DESCRIPTION

FIG. 1 shows, in block diagram form, aportion of one exemplaryembodiment of a measuring tool 100 on which the present invention may bedeployed. Measuring tool 100 is deployable within a subterraneanborehole to investigate the propagation characteristics of anelectromagnetic wave passing through the surrounding earth formation.Measuring tool 100 may be advantageously employed to determine anattribute of either a section of the borehole or a section of thesurrounding earth formation, such as, for example, its resistivity orits dielectric constant.

Measuring tool 100 comprises a logging collar 110, which, in FIG. 1 isillustrated as essentially an elongated steel shaft. Logging collar 110is adapted to be positioned within a borehole 140. In one embodiment,logging collar 110 is adapted to be in the drill string close to thedrill bit to provide measurement-while-drilling orlogging-while-drilling. Measuring tool 100 is advantageously adapted toprovide accurate measurements under a wide range of ambient temperaturesand adverse conditions commonly found while drilling within asubterranean borehole.

In a first exemplary embodiment, logging collar 110 includes twotransmitters TX1 122, TX2 128 and a pair of differential receiverantennae RX1 124 and RX2 126. TX1 122, TX2 128, RX1 124 and RX2 126 eachcomprise a coil that is wound with one or more turns on an insulatingsurface within in a recess circumferential to logging collar 110. In onesuitable embodiment TX1 122 and TX2 128 are spaced about 4 to 8 ftapart, axially on logging collar 110. TX1 122 and TX2 128 areindividually controllable to selectively radiate an electromagnetic wavecomprising a plurality of predetermined interrogation frequencycomponents. In the exemplary embodiment, two interrogation frequencies500 Khz and 2 Mhz are employed. In another exemplary embodiment, a thirdinterrogation frequency of 1 Mhz is also employed. In the exemplaryembodiment, RX1 124 and RX2 126 are spaced about 10 inches apart axiallyon logging collar 110 and centered between TX1 122 and TX2 128. Receiverantenna RX1 124 and RX2 126 are each adapted to detect bands offrequencies centered on each of the plurality of interrogationfrequencies. A pick-up signal is generated by each receiver antenna RX1124 and RX2 126 representing the phase shift and/or attenuation of theinterrogation frequency components as the electromagnetic wave passesbetween the differential pair of receiver loop antennae RX1 124 and RX2126.

One skilled in the art will recognize that the embodiments of thepresent invention are not limited to logging-while-drilling ormeasurement-while-drilling applications, and may be extended to othertypes of applications, such as, for example, wire line systems.Embodiments will further be appreciated to be adaptable for a wide rangeof logging collar geometries and axial spacing for receivers andtransmitters, as well as a wide range of interrogation frequencies. Inaddition, embodiments of the present invention may include tools havinga single transmitter.

FIG. 2A depicts a schematic block diagram of an exemplary embodiment ofan antenna receiver system 200, which is suitable for the measuring tool100 of FIG. 1 having receiver antennae RX1 124 and RX2 126. Antennareceiver circuit 200 includes antenna 202, which, in the exemplaryembodiment, is a loop antenna corresponding to either of RX1 124 or RX2126, having about a 6 inch diameter, 6 turns, and an inductance ofapproximately 18 μh. Antenna 202 is coupled in series with the primarywinding of a step-up transformer 220 and tuning network 204, so as togenerate a pick-up signal across the secondary winding of transformer220 (in the form of a voltage differential across nodes 235 and 238) inresponse to an electromagnetic wave passing antenna 202. The secondarywinding of transformer 220 is coupled to a preamplifier circuit 206 thatgenerates an amplified version of the pick-up signal on output nodes 281and 282. In an exemplary embodiment, output nodes 281 and 282 couplethrough a 4 to 6 foot coaxial cable (having a characteristic impedanceof 50 to 100 ohms) to an external amplifier circuit, which is not shown.A microprocessor-based data acquisition system (not shown) samples thepreamplifier output signal to determine the relative amplitude andrelative phase of the electromagnetic wave at each of the plurality ofinterrogation frequencies.

Still referring to FIG. 2A, step-up transformer 220 serves to amplifythe signal received from antenna 202 on the primary winding to provide apick-up signal on the secondary winding. Conductors 221 and 224 eachcouple to an opposite end of the primary winding of step-up transformer220. Conductor 221 couples to a first end of antenna 202 and conductor221 couples to the other end of antenna 202. The primary winding oftransformer 220 is split at nodes 222 and 223 to define a first portionand a second portion of the primary winding. Tuning network 204 couplesto nodes 222 and 223 to be in series with antenna 202 and the primarywinding of step-up transformer 220. In an exemplary embodiment, step-uptransformer 220 is of common bobbin construction and is comprised of astandard core from TDK™ part number PC44ER11/5-Z; each of the twoprimary winding comprises of 5 turns of #32 wire and the secondarywinding comprises 32 turns of #34 wire. One skilled in art willrecognize that although it is advantageous to split the primary side ofthe transformer into a first and second portion to balance the effectsof stray capacitance and inductance inherent in the components andconductors, other suitable embodiments are available, such as atransformer that is not split, or that has a center tap on the primarywinding.

Tuning network 204 cooperates with antenna 202 to achieve “simultaneoustuning” at a plurality of interrogation frequencies. “Simultaneoustuning” results in a pick-up signal that is strong for the bandsapproximately centered at each of the plurality of interrogationfrequencies and attenuated for the other frequencies (i.e. out-of-bandfrequencies).

The exemplary embodiment of antenna receiver circuit 200 isadvantageously configured with antenna 202 and tuning network 204coupled in series to achieve “simultaneous series tuning” at a pluralityof interrogation frequencies. This is implemented by providing for theimpedance of the antenna 202 and tuning network 204 combination to benegligible (advantageously close to zero) for a narrow band around eachof the interrogation frequencies and, at the same time, for theimpedance to rise for out-of-band frequencies so that reception by theantenna 202 effectively excludes (or substantially attenuates)out-of-band frequencies. The exemplary embodiment of antenna receivercircuit 200 provides for negligible impedance of about 10 ohms, which issufficiently low for most applications, although the invention is notlimited in this regard. One skilled in the art will understand that thebands are substantially centered on each of the interrogationfrequencies and are preferably as narrow as possible to advantageouslyprovide a high signal-to-noise ratio when in-band. However, theinvention is not limited to any particular in-band bandwidth, and thewidth of the narrow bands may be selected to specifically attenuateparticular frequencies anticipated in particular embodiments to causeinterference with the electromagnetic wave measurements at theinterrogation frequencies.

FIGS. 2B, B2, 2C and 2D illustrate simultaneous series tuning for twoexemplary embodiments. FIG. 2B shows Z_(A) selected to represent theimpedance of the antenna 202 and tuning network 204 combination shown onFIG. 2A. FIG. 2C and 2D are plots of impedance Z_(A) versusinterrogation frequency approximated by an analog circuit simulator. Thevertical scale represents impedance linearly. The horizontal scale islogarithmic and represents the frequency range of 100 Khz through 10Mhz. FIG. 2C shows the impedance Z_(A) versus frequency response for anexemplary embodiment of tuning network 204 that provides simultaneousseries tuning at the two interrogation frequencies of 500 KHz and 2Mhz.FIG. 2D depicts the impedance Z_(A) versus frequency response for analternate embodiment of a tuning network 204 that provides simultaneousseries tuning at the three interrogation frequencies of 500 KHz, 1 Mhzand 2 Mhz. As shown in FIGS. 2C and 2D, the impedance Z_(A) isnegligible for a narrow band centered on each of the interrogationfrequency and rises for the out-of-band frequencies.

It will be understood that the level of source impedance of the antenna202 and tuning network 204 combination is of interest. Consistent withthe invention, a source impedance that will provide sufficientattenuation so as to effectively exclude out-of-band frequencies isdependent on the load impedance. For example, with further reference toFIG. 2A, if a preamplifier 206 exhibits an input impedance of 3600 ohmsat a particular out-of-band frequency, then a step-up transformer 220with a turns ratio of 5:5:32 (i.e. 1 to 3.2), will present a loadimpedance of about 350 ohms to the antenna 202 and tuning network 204combination. Thus, when the combination of antenna 202 and tuningnetwork 204 has a source impedance of 350 ohms, the pick-up signalacross the secondary windings of transformer 202 is attenuated by 6 db(i.e. signal is reduced by a factor of 2). Attenuation for out-of-bandfrequencies can be designed to be much higher, as illustrated by anotherexemplary embodiment shown on FIG. 21.

In accordance with another aspect of the invention, the impedance of theload presented to the combination of antenna 202 and tuning network 204may be tailored to reduce the secondary electromagnetic radiation fromantenna 202 to below a predetermined value. Generally speaking, it isunderstood that the magnitude of a secondary magnetic field radiated byan antenna (such as antenna 202 on FIGS. 2A, 2B and 2G) is proportionalto the frequency for a given circulating current. For this reason, theload impedance may be tailored to be higher for the higher interrogationfrequencies, as compared to the lower interrogation frequencies in orderto reduce measurement error components due to antenna differential pairmagnetic cross-coupling. The particular interrogation frequencies may beselected for each application based on the attributes of a formationbeing investigated. The impedance characteristics of the load presentedto antenna 202 and tuning network 204 may be tailored specifically foreach application based, for example, on the selected interrogationfrequencies, the source impedance characteristics, and desired gain ofthe received pick-up signal. The invention is not limited in regard toselection of load impedance. For out-of-band frequencies, preamplifiercircuit 206 may present a low impedance load to the antenna 202 andtuning network 204 without affecting the measurements.

FIG. 2E shows a block diagram of an embodiment of a preamplifier 206that has an input impedance that presents a load impedance to tuningnetwork 204 and antenna 202. In FIG. 2E, the load impedance is tailoredto reduce the secondary electromagnetic radiation from antenna 202 to bebelow a predetermined level. Zp is selected to illustrate the inputimpedance characteristics of preamplifier 206. FIG. 2F is a plot of Zpderived by shorting the signal across nodes 222 and 223 and simulatingthe impedance observed across nodes 221 and 224. FIG. 2F shows that in aparticular embodiment, at 2 Mhz the impedance Zp is about 350 ohms.

FIGS. 2G through 21 are illustrative of the low level of circulatingcurrents in antenna 202 achieved, in the exemplary embodiments, whilemaintaining serviceable gain at each of the plurality of interrogationfrequencies. FIG. 2G shows antenna receiver system 200 with theembodiment of tuning network 204 that has the characteristics shown inFIG. 2C, and the preamplifier that has the characteristics shown in FIG.2F. FIG. 2H is a plot of impedance Zp versus frequency response for theantenna receiver system 200 shown in FIG. 2G. Zp is selected, forillustration purposes, by breaking the circuit at node 224 andsimulating the impedance Zp versus frequency response to indicate thelevel of circulating currents in antenna 202. FIG. 21 is a simulatedplot approximating the gain Vout/Vin, as shown in FIG. 2G with node 224coupled to antenna 202 (as shown by the dotted line). The gain achievedfor each of the two interrogation frequencies of 50OMhz and 2Mhz isshown, as well as the relative attenuation of the other frequencies.

Additional considerations may be required in the tailoring of theantenna source/load impedance. The antenna load impedance primarilycomprises of the “magnetizing” impedance of transformer 220 and theimpedance presented by preamplifier circuit 206. These two impedancesare in parallel. Step-up transformer 200 may have a magnetizingimpedance that varies with frequency as well as temperature. In order tomaintain thermal stability of the combined impedance within the widerange of extreme temperature conditions that may be encountered within aborehole, the impedance of the preamplifier may advantageously beselected to be lower than that of the transformer at the integrationfrequencies.

The conductors coupling antenna 202 to preamplifier circuit 206 arepreferably implemented to be short, in order to minimize the load onantenna 202. In actual application, however, the tuning network 204 andpreamplifier circuit 206 may effectively negate this minimized loading,as the preamplifier, in the exemplary embodiments, may need to belocated within a few inches of the receiver antenna.

In accordance with another aspect of the present invention, exemplaryembodiments may employ a substantial mismatch in the impedance ofantenna 202 and its load to allow embodiments of antenna 202 to compriseof more than one turn. For example, a six turn antenna of about a 6 inchdiameter has a characteristic inductance of about 18 μH. It is generallyunderstood that multiple turn antenna generally provide a strongerpick-up signal than can be achieved with a single turn antenna ofsimilar geometries. The invention is nonetheless not limited in thisregard, and other embodiments may advantageously provide receivercircuits having an antenna with a single turn, two or more turns, or adifferent geometry.

FIG. 3 is a detailed schematic of an exemplary embodiment of a tuningnetwork 300 that is suitable for the tuning network 204 shown in FIG. 2.Tuning network 300 is configured, in the illustrated embodiment, toprovide simultaneous series tuning at the two predeterminedinterrogation frequencies of 500Khz and 2Mhz, although it will beunderstood that the invention is not limited in this regard. Tuningnetwork 300 on FIG. 3 for loop antenna 202 on FIG. 2A is coupled inseries to the primary winding of step-up transformer 220 and antenna202. One end of tuning network 300 couples to node 222 and on the otherend to node 223, as shown on both FIGS. 2A and 3. Tuning network 300 iscomprised of inductor L1 360 and capacitor C1 362, which are coupledtogether in parallel. The parallel combination is coupled in series withcapacitor C2 368.

As noted earlier in the discussion of FIG. 2A, antenna 202 and tuningnetwork 204 cooperate to achieve simultaneous series tuning by providingfor a combined impedance that effectively excludes (or substantiallyattenuates) reception at frequencies outside each of the narrow bands offrequencies around the plurality of interrogation frequencies. For agiven antenna inductance L_(Ant), the following simultaneous equationsare satisfied for two operating frequencies (it is assumed that the realpart of the various impedances are negligible):Z _(Ant1) +Z _(C2) +Z _(C1L1)=0 at Frequency F1  (1)Z _(Ant2) +Z _(C2) +Z _(C1L1)=0 at Frequency F2  (2)where Z_(Ant)>0, Z_(c2)<0 and represents the reactance of antenna 202and the reactance of tuning capacitor C2 368; andZ_(C1L1)=(Z_(C1)*Z_(L1))/(Z_(C1)+Z_(L1)) and represents the netreactance of the parallel combination of L1 360 and C1 362.

In an exemplary embodiment, the interrogation frequencies are selectedto be about 500 Khz and 2 Mhz and antenna 202 has a characteristicinductance L_(Ant) of 18 μH; C1 362 is selected to be 620 pfd; C2 368 isselected to be 1250 pfd; and inductance of L1 360 is selected to be 47μH. In addition, the effective inductance variation of antenna 202 atthe different operating frequencies preferably is taken into account.Inductance variation with frequency may occur if the self-resonantfrequency of the loop antenna 202 is not significantly higher than thehighest desired operating frequency.

Typically there is also a point of maximum impedance between the twominima. Selection of inductor L1 360 has some influence on the frequencypoint of maximum impedance located between the two minima, and also onthe impedances of the out-of-band frequencies; however, the values of C1362 and C2 368 are unique for a given value of L1 360 and the value ofL_(Ant)

FIG. 4 shows a detailed schematic of an alternative embodiment of atuning network 400 that is suitable for a tuning network 204 shown inFIG. 2. Tuning network 400, in an exemplary embodiment, is configured toprovide simultaneous series tuning at the two selected interrogationfrequencies of 500 Khz and 2 Mhz. Tuning network 400 for loop antenna202 is coupled in series to the primary winding of step-up transformer220 and antenna 202. One end of tuning network 400 couples to node 222and on the other end to node 223. Tuning network 400 comprises of theparallel configuration of two circuits: the first circuit comprises aninductor L1 470 and a capacitor C1 472 that are coupled together inseries and the second circuit comprises an inductor L2 474 and acapacitor C2 476 that are coupled together in series. The inductance ofthe antenna 202 and tuning network 204 is preferably negligible(advantageously as close to zero) at each interrogation frequency toachieve simultaneous series tuning. Accordingly, the following twoequations are solved for each of the two interrogation frequencies F1and F2 to determine the particular values of inductance for L1 470 andL2 474 and particular values of capacitance for C1 472 and C2 476:$\begin{matrix}{{Z_{Ant} + \frac{( {Z_{L\quad 1} + Z_{C\quad 1}} )( {Z_{L\quad 2} + Z_{C\quad 2}} )}{Z_{L\quad 1} + Z_{C\quad 1} + Z_{L\quad 2} + Z_{C\quad 2}}} = {0\quad{at}\quad{frequency}\quad F\quad 1}} & (3) \\{{Z_{Ant} + \frac{( {Z_{L\quad 1} + Z_{C\quad 1}} )( {Z_{L\quad 2} + Z_{C\quad 2}} )}{Z_{L\quad 1} + Z_{C\quad 1} + Z_{L\quad 2} + Z_{C\quad 2}}} = {0\quad{at}\quad{frequency}\quad F\quad 2}} & (4)\end{matrix}$

With reference to FIG. 5, one skilled in the art will recognize that thepresent invention is not limited in its series tuning aspect to anyparticular number of interrogation frequencies. For example, FIG. 5shows a tuning network 500 which provides simultaneous series tuning atthree interrogation frequencies of 500 Khz, 2 Mhz and I Mhz. One end oftuning network 500 couples to node 222 and on the other end to node 223.Tuning network 500 comprises a parallel combination including inductorL1 560 and capacitor C1 562, and a parallel combination including L2 563and C3 565. The two parallel combinations are coupled in series togetherand with capacitor C2 566. Simultaneous series tuning is achieved byproviding an equation representing the combined impedances of the tuningnetwork 500 and antenna 202 and solving the equation to where thecombined impedance is zero at each of the three interrogationfrequencies.

Antenna 202 and tuning network 204 cooperate to achieve simultaneousseries tuning by providing for a combined impedance that sufficientlyattenuates reception by antenna 200 to effectively exclude reception atfrequencies outside each of the predetermined narrow bands offrequencies around the interrogation frequencies. This approach leads tothree equations that are solved for a 18 μH antenna 202 at the threeinterrogation frequencies of 500 Khz, 1 Mhz, and 2 Mhz. L1 and L2 areeach selected to be 10 μH, C2 is selected to equal 2400 pfd, C3 isselected to equal 1100 pfd and C1 is selected to equal 3300 pfd.

FIG. 6A depicts a high-level block diagram of a circuit suitable for thepreamplifier circuit 206 depicted in the schematic of FIG. 2. In FIG.6A, the impedance of the load to antenna 202 and tuning network 204 isadvantageously tailored specifically for a particular application basedon the selected plurality of interrogation frequencies. This objectiveis to reduce the effect of the secondary electromagnetic field to belowa predetermined acceptable level. A pick-up signal from the secondarywinding of step-up transformer 220 is received on input nodes 235 and238. An amplified version of the pick-up signal is generated on outputnodes 281 and 282. Preamplifier circuit 206, in this exemplary circuit,is comprised of an amplifier circuit 616, with a positive gain, and apositive-feedback path 614 for providing feedback that is a function offrequency to tailor the input impedance of the preamplifier circuit 206.

It will be appreciated that the present invention is also able to reducethe disadvantageous effectives of cross-coupling. A beneficial result ofmaximizing the load impedance presented to the receiver loop antenna 202is that the secondary electromagnetic field radiated by receiver loopantenna 202 is minimized for the interrogation frequencies, therebydiminishing the cross-coupling error in any other receiver antenna thatis sufficiently close to be measurably affected by cross-coupling. Inaddition, the feedback mechanism may be selected specifically tomaintain thermal stability of the combined load impedance of the step-uptransformer 202 and the preamplifier circuit 206.

FIG. 6B shows a detailed schematic of an exemplary embodiment that issuitable for a preamplifier circuit 206. Node 238 and node 281 arecoupled to a common ground. A pick-up signal from the secondary windingof step-up transformer 220 is received on node 235 and an amplifiedversion of the pick-up signal is generated on amplifier output node 282.Amplifier circuit 616 is implemented with operational amplifier 652 inconjunction with resistors R7 644, R13 646, R2 654 and C4 656 to receivethe pick-up signal on node 235 and to generate an amplified version ofthe pick-up signal on output node 282. A positive-feedback path 614 isprovided by the series network, linking the output node 282 to thepositive input of operational amplifier 652. Positive feedback path 614comprises resistor R7 644, capacitor C1 9642, resistor R14 648, and theparallel combination of R2 654 and C4 656.

In one embodiment of the circuit of FIG. 6B, operational amplifier 652may be deployed in the form of an integrated circuit available fromElantec, part no. EL2125. Resistor R2 654 and capacitor C4 656 arecoupled together in parallel and each couple on one end to output node282 and on the other end to the negative input to operational amplifier652. In this exemplary embodiment, resistor 654 is 100 ohms andcapacitor 656 is 47 pf. The negative input of operational amplifier 652is coupled to ground through resistor R6 650, which is 10 ohms.Resistors R7 644 and R13 646 are each 750 ohms and are each coupled onone end in series to each other. One end of the resistor pair R7 644 andR13 646 is coupled to the positive input of operational amplifier 652and the other end of resistor pair R7 644 and R13 646 is coupled toground. Capacitor C19 642, selected to be 1000 pf, is coupled in seriesto resistor R14 648, selected to be 249 ohms. The series combination ofcapacitor C 19 642 and resistor R14 648 are coupled on one end to thenegative input of operational amplifier 652 and on the other end to thenode that couples to both of the resistors R7 644 and R13 646.

In the exemplary embodiment depicted in FIG. 6B, preamplifier circuit206 is specifically tailored for interrogation frequencies 500 Khz and 2Mhz. The addition of capacitor C19 642 and resistor R14 648 provide thepositive-feedback path 614 shown in FIG. 6A, which increases the inputimpedance of the preamplifier circuit 206 at 2 Mhz and to a lesserdegree at 500 Khz. This particular configuration of a positive-feedbackpath, also known as “boot-strapping,” results in the effective loadimpedance of the pre-amplifier 206 being increased by more than a factorof two at 2 MHz. In addition, the impedance at the lower operatingfrequency has also been increased significantly. The capacitance valueof C19 642 and resistance value of R14 648 are selected to increase ordecrease the amount of positive feedback at the plurality ofinterrogation frequencies to reduce the magnitude of the secondaryelectromagnetic field to below a predetermined acceptable level. Notehowever that, in some applications, the shunt admittance of thetransformer, if dominant, may tend to create thermal instability of theinput impedance presented by the step-up transformer 220 andpreamplifier circuit 206. In such situations, thermal instabilityconsiderations may set an upper boundary for efforts to increase theinput impedance of the preamplifier circuit 206.

With further reference to the exemplary embodiment depicted in FIG. 6B,the impedance of the preamplifier circuit 206 without the presence ofcapacitor C19 642 and resistor 648 R14 would be approximately 1500 ohms,which is the sum of the two 750 ohm resistors. The addition of capacitorC19 642 and resistor R14 648 raises the input impedance of thepreamplifier circuit from 1500 ohms to approximately 3500 ohms at the 2MHz interrogation frequency. At interrogation frequency 500 Khz, theimpedance of the preamplifier circuit is about 2600 ohms and the loadimpedance presented to the antenna is about 146 ohms due to the step-uptransformer. Accordingly, the secondary electromagnetic field and errordue to cross-coupling are kept below a predetermined value for each ofthe interrogation frequencies.

It will be appreciated that a higher value of impedance for thepreamplifier circuit 206 could be achieved by reducing the turns ratioof the step-up transformer. However, such a reduction in the turns ratiowould require additional amplification by pre-amplifier 206, therebypotentially causing a decrease in the signal to noise ratio of theantenna receiver 200.

FIG. 7 depicts a detailed schematic diagram of an alternative exemplarycircuit that would be suitable for the preamplifier 206 shown in FIG. 2.A series of LC circuits are provided to tailor the input impedance for aselected plurality of interrogation frequencies so as to minimizecross-coupling between receiver antennae. The positive input tooperational amplifier 752 is coupled to the secondary winding of step-uptransformer 220 through node 235 to receive the pick-up signal fromantenna 202. The output of the operational amplifier connects to outputnode 282. Resistor R2 754 and capacitor C4 756 are coupled together inparallel and each couple on one end to output node 282 and on the otherend to the negative input to operational amplifier 752. In thisexemplary embodiment, resistor R2 754 is 100 ohms and capacitor C4 756is 47 pf. The negative input of operational amplifier 752 is coupled toground through 10 ohm resistor R6 750. The positive input to theoperational amplifier 752 connects to ground through a DC blockingcapacitor C3 780 and a circuit comprising two LC circuits coupled inseries. The first LC circuit comprises inductor L1 772 coupled inparallel to capacitor C1 776. The second LC circuit comprises inductorL2 774 coupled in parallel to capacitor C2 778. The values of L1 772 andC1 776 are advantageously selected according to the following equationfor the first interrogation frequency F1: $\begin{matrix}{{F\quad 1} = \frac{1}{2\pi\sqrt{L\quad 1C\quad 1}}} & (5)\end{matrix}$

The values of L2 and C2 are advantageously selected according to thefollowing equation for the second interrogation frequency F2:$\begin{matrix}{{F\quad 2} = \frac{1}{2\pi\sqrt{L\quad 2C\quad 2}}} & (6)\end{matrix}$

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalternations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims.

1. A loop antenna circuit for use in a borehole, comprising: a loopantenna disposed to generate, in response to an electromagnetic wave, apick-up signal on an output node thereof, the pick-up signalrepresentative of, for each of a plurality of interrogation frequencies,relative magnitude and relative phase of the electromagnetic wave; and atuning network electrically connected to the loop antenna, the tuningnetwork configured to tune the loop antenna such that a combined sourceimpedance of the loop antenna and the tuning network varies withfrequency, the combined source impedance having minima within apreselected band of frequencies about each of the interrogationfrequencies.
 2. The loop antenna circuit of claim 2, wherein thecombined source impedance has at least one maximum at a frequencyoutside the preselected bands.
 3. The loop antenna circuit of claim 1,further comprising: a preamplifier circuit with an input nodeelectrically connected with the loop antenna and the tuning network, thepreamplifier circuit having an input impedance that is sufficiently highto reduce secondary radiation from the loop antenna to below apredetermined value at at least one of the plurality of interrogationfrequencies.
 4. The loop antenna circuit of claim 3, wherein the inputimpedance of the preamplifier circuit exceeds the combined sourceimpedance of the loop antenna and the tuning network at each of theplurality of interrogation frequencies.
 5. The loop antenna circuit ofclaim 3, wherein the preamplifier circuit is electrically connected withthe loop antenna through a step-up transformer.
 6. The loop antennacircuit of claim 1, wherein, the tuning network and loop antenna areelectrically connected in series to one another.
 7. The loop antennacircuit of claim 6, wherein the tuning network comprises first andsecond LC circuits, each of which includes an inductor and a capacitorelectrically connected to one another in series, the first and second LCcircuits electrically connected to one another in parallel to form athird LC circuit, the third LC circuit electrically connected in serieswith the loop antenna.
 8. The loop antenna circuit of claim 6, whereinthe tuning network comprises an LC circuit having an inductor and afirst capacitor electrically connected in parallel, the LC circuit and asecond capacitor electrically connected in series with the loop antenna.9. The loop antenna circuit of claim 6, wherein the tuning networkcomprises first and second LC circuits, each of which includes aninductor and a capacitor electrically connected to one another inparallel, the first and second LC circuits connected to one another inseries to form a third LC circuit, the third LC circuit and a capacitorelectrically connected in series with the loop antenna.
 10. A receiverfor a downhole tool, the receiver comprising: a loop antenna includingat least a single turn of antenna wire deployed about a tool housing,the loop antenna disposed to receive an electromagnetic wave in aborehole; a tuning network electrically connected in series with theloop antenna, the tuning network configured such that a series impedanceof the loop antenna and the tuning network is less than the impedance ofthe loop antenna in a preselected frequency band about at least oneinterrogation frequency; a preamplifier circuit having an inputelectrically connected in series with the loop antenna and the tuningnetwork, the preamplifier circuit having an input impedance selectedsuch that a series impedance of the loop antenna, the tuning network,and the preamplifier circuit is sufficiently high to reduce secondaryradiation from the loop antenna to below a predetermined value at the atleast one interrogation frequency.
 11. The receiver of claim 10, whereinthe loop antenna comprises at least six turns of antenna wire.
 12. Thereceiver of claim 10, wherein the input of the preamplifier circuitcomprises a step-up transformer.
 13. The receiver of claim 10, whereinthe input impedance of the preamplifier circuit exceeds a sourceimpedance of the loop antenna and the tuning network at the at least oneinterrogation frequency.
 14. The receiver of claim 10, wherein thetuning network comprises an LC circuit having an inductor and a firstcapacitor electrically connected in parallel, the LC circuit and asecond capacitor electrically connected in series with the loop antenna.15. The receiver of claim 10, wherein the series impedance of the loopantenna and the tuning network is greater than the impedance of the loopantenna at frequencies outside said preselected bands.
 16. A loggingwhile drilling tool comprising: a housing adapted to be coupled to adrill string and deployed in a subterranean borehole; a transmitterdeployed on the housing, the transmitter disposed to transmit anelectromagnetic wave into the borehole, the electromagnetic waveincluding a plurality of interrogation frequency components; first andsecond, longitudinally spaced loop antennas disposed to receive theelectromagnetic wave, each of the loop antennas having at least a singleturn of antenna wire deployed about the housing; a tuning networkelectrically connected in series to the first loop antenna, the tuningnetwork configured such that a series impedance of the loop antenna andthe tuning network is less than the impedance of the loop antenna in apreselected frequency band about each of said plurality of interrogationfrequencies.
 17. The logging while drilling tool of claim 16, furthercomprising: a preamplifier circuit having an input electricallyconnected in series with the first loop antenna and the tuning network,the preamplifier circuit having an input impedance selected such that aseries impedance of the loop antenna, the tuning network, and thepreamplifier circuit is sufficiently high to reduce cross coupling fromthe first loop antenna to the second loop antenna to below apredetermined value at at least one of said plurality of interrogationfrequencies.
 18. The logging while drilling tool of claim 17, whereinthe input impedance of the preamplifier circuit exceeds the sourceimpedance of the loop antenna in the tuning network for each of saidplurality of interrogation frequencies.
 19. The logging while drillingtool of claim 16, wherein the tuning network comprises an LC circuithaving an inductor and a first capacitor electrically connected inparallel, the LC circuit and a second capacitor electrically connectedin series with the loop antenna.
 20. The logging while drilling tool ofclaim 16, wherein the series impedance of the loop antenna and thetuning network is greater than the impedance of the loop antenna atfrequencies outside said preselected bands.